Reduction in lubricant pickup by bias voltage between slider and disk surfaces

We studied the effect of bias voltage between slider and disk surfaces to reduce lubricant pickup by the slider. A perfluoropolyether (PFPE) lubricant film, which is coated on the disk surface, has been considered to be charged to a negative voltage by the airflow on the rotational disk surface. Because the PFPE lubricant film is negatively charged, the lubricant pickup should be reduced by a bias voltage with a negative voltage on the slider surface. We confirmed changes in the lubricant pickup in a lubricant pickup test conducted at different bias voltages. A positive voltage of the slider accelerated the lubricant pickup, whereas a negative voltage reduced it.     

Vacuum vapor deposition of PFPE molecules on CHxNy and CHxFy amorphous carbon surfaces

According to the demand of increasing storage density for the magnetic data storage, a contact recording system is proposed, in which the head constantly makes contact with the disk surface during read/write cycles. In this system a stronger lubricant film for head disk interface (HDI) is required. In a conventional dip-coating method, which is carried out under atmospheric conditions, however, a fresh diamond like carbon (DLC) surface adsorbs several contaminations from the air and a strong lubricant layer is difficult to form on the DLC surface. In this study, a DLC layer is coated on a Si substrate by a plasma chemical vapor deposition (PCVD) method, then the DLC-coated Si sample is transferred to another vacuum chamber without being exposed to the atmosphere, and a perfluoropolyether (PFPE) lubricant film is vapor-deposited on the sample. The sample is analyzed by XPS and compared to the surface on which the PFPE lubricant is dip-coated. It is noted that both samples are rinsed after the coatings and the loose PFPE molecules are washed away. A thicker bonded layer of PFPE is formed on the DLC surface treated by vapor deposition. A tribological wear test is also carried out using a pin-on-disk tribotester. Both results show that the tribological characteristics are improved by the vapor deposition of PFPE molecules compared to the dip-coating method.     

Tribological characteristics of novel branched perfluoropolyether lubricant films in hard disk drives

In this study, the basic tribological characteristics of novel branched perfluoropolyether (PFPE) lubricant films such as TA-30 and QA-40 were examined. Their surface free energy characteristics and adhesive and friction forces were investigated using an atomic force microscope. The interactions between the lubricant molecules and the water molecules were also examined by monitoring the changes in the contact angle of distilled water on the test lubricant films. The interactive forces such as the adhesive and friction forces of a film that is approximately one monolayer thick were found to be strongly dependent on the conformation of the lubricant molecules on diamond-like carbon thin films. In addition, the TA-30 and QA-40 lubricant molecules appeared to interact with the water molecules more actively than conventional Ztetraol2000 molecules. These results afforded fundamental insight into the tribological performance of novel branched PFPE lubricants in the head-disk interface.     

Lubrication of advanced metal evaporated tape using novel perfluoropolyether lubricants

In order to reduce the failure of advanced metal evaporated (AME) tape, novel perfluoropolyether (PFPE) lubricants, Z-TETRAOL and A20H, were deposited on an unlubricated AME tape. The degradation mechanisms of the PFPE lubricants applied on AME tape were studied using a mass spectrometer in high vacuum. The durability of various lubricants was compared using pin-on-disk experiments in high vacuum and using an accelerated tape tester in ambient air. It was found that PFPE lubricants were mainly degraded by tribochemical reaction and mechanical shear. The durability of Z-TETRAOL and A20H are higher than conventional PFPE lubricant and lubricant on experimental tape. By using ultraviolet light treatment and by applying a stearic acid overcoat, the durability of PFPE lubricants could be improved.     

Reduction in total surface area by the develpment of microdroplets during dewetting

To meet the demand of an increasing storage density, the lubricant film for the head disk interface (HDI) needs to be thinner and stronger. In recent years, a new head/disk system, such as the contact type, has been proposed. It is reported that PFPE Zdol coated on a magnetic disk is dewetted and microdroplets are formed due to polar interactions. This makes a flying magnetic head unstable, therefore, the physics and chemistry of the dewetting phenomenon are topics of current interest. We investigated the formation and development of microdroplets using an atomic force microscope (AFM) and an optical microscope. First, we observed the disk surface coated with PFPE Zdol by AFM. From the cross section images of the microdroplets, we found that the microdroplets had a shape similar to a sphere. With this finding, we estimated the contact angle of the microdroplets in each image. The results showed that the contact angle of the microdroplet gradually decreased with time, which indicated the existence of a PFPE thin film in the dewetted area. The thickness of the PFPE film in the dewetted area was then measured using an elliposometer. Next, we investigated the variation in the number and the average diameter of the microdroplets during dewetting using images observed by the optical microscope. The total surface area change was also calculated from the observed results, and it was found that the total surface area, namely the sum of the microdroplet surfaces and dewetted area, was reduced by the development of the microdroplets.     

Lubricant bonding, chemical structure, and additive effects on tribological performances at head–disk interfaces

 The lubricant film for head/disk application consists of bonded fraction and unbonded (mobile) fraction. It is well known that the mobile fraction of the lubricant film can replenish the surface sites where the lubricant film was depleted, thus, the surface wear is postponed or alleviated. With a continuous decrease in the head disk spacing, however, too much mobile fraction of lubricant may cause head slider lubricant pick-up, and deteriorate the interface. Two perfluoropolyether (PFPE) lubricants of Z-tetraol and Z-DOL are discussed in this paper. Lubricant Z-tetraol is characteristic of stronger bonding to a carbon overcoat, lower vapor pressure, and higher thermal stability but less mobility than Z-DOL. It is found that, for CSS (contact-start-stop) durability, the interfaces with Z-tetraol show no worse in performance than those with Z-DOL, and less head slider lubricant pick-up on those with Z-tetraol. Based on the above-mentioned, it is possible that the interfaces relying more on the lubricant bonding strength and chemical structure stability are more beneficial to tribological performances than those relying more on the lubricant replenishment. The effects of lubricant additive X1P mixed to Z-tetraol, and Z-DOL, respectively are also studied on tribological performances. Stiction, CSS durability, and head slider lubricant pick-up are discussed among lubricants Z-tetraol, Z-DOL, Z-tetraol/X1P and Z-DOL/X1P. Statistical t-test, F-test, and Weibull analyses are applied to CSS data to differentiate CSS durability performances. Additive X1P is found to enhance CSS durability for both lubricants. Lubricant Z-tetraol/X1P is recommended for the best tribological performances, followed by Z-DOL/X1P, Z-tetraol, and Z-DOL. Received: 7 August 2001/Accepted: 11 December 2001 Authors would like to thank Vidya K. Gubbi, Youmin Liu, and Gunter P. Barth at Seagate, Fremont, CA, for media supplies, lubricant properties, and head slider lubricant pick-up measurements. Authors also thank Gunter P. Bath, Frank Chang, Roger Y. Shih, Hamid R. Saman, Caroline Tjengdrawira and Sam Liang at Seagate for fruitful discussions.     

Ultraviolet bonding of perfluoropolyethers to carbon surfaces investigated using quantum chemical methods

For improving the tribological performance of hard disk drives, nanometer-thick perfluoropolyether (PFPE) lubricant films are generally treated with ultraviolet (UV) irradiation to bond them to the carbon overcoats of the disks. By modeling UV irradiation as an electron emission and attachment process, we investigate the UV bonding of nonfunctional PFPE Z and functional PFPE Zdol to hydrogenated and nitrogenated carbon surfaces with quantum chemical methods. Our calculation results show that, upon electron attachment, Z dissociates at its main chain to two fragments terminated by CF₂CF₂ and CF₂O groups, whereas Zdol dissociates to a hydrogen fluoride and a fragment. The perfluoromethoxy oxygen in one of the Z fragments and the carbon radical and the hydrogen-truncated end group in the Zdol fragment interact strongly with sp² and oxidized sites on carbon surfaces. Imine moieties on the CNx surface also contribute considerably to the UV bonding of Zdol.     

Depletion of monolayer liquid lubricant films induced by high-frequency pulsed-laser heating in thermally assisted magnetic recording

In this study, lubricant depletion due to high-frequency pulsed-laser heating was investigated for lubricant films with thicknesses of both more than and less than one monolayer. A conventional lubricant, Zdol2000, was used. It was found that the critical temperature at which the lubricant begins to deplete owing to laser heating was strongly dependent on the lubricant film thickness. In the case in which the thickness of the lubricant film was less than one monolayer, this temperature was approximately 170?°C higher than it was when the thickness was more than one monolayer. To analyze the lubricant depletion mechanism, we examined the tested lubricant film using temperature programmed desorption (TPD) spectroscopy. It was found that the lubricant depletion characteristics due to laser heating could be explained using the experimental TPD results for the tested lubricant film, and that the depletion mechanism involves the desorption or decomposition of the lubricant molecules, which interact with the diamond-like carbon thin films when the lubricant film thickness is less than one monolayer. Further, the results of TPD and of a thermogravimetric analysis (TGA) of the lubricant were compared. The thermal robustness of the ultra-thin liquid lubricant films was found to be greater than that of the bulk lubricant materials.     

Ultraviolet irradiation of lubricant and additives thin films to reduce wear to the magnetic head

This paper examined the effects of using 1 nm thickness lubricant thin film combined with additives and deep ultraviolet (UV) irradiation at 185 nm wavelength on the magnetic hard disk to the wear of the magnetic head during contact. Different types and amount of additives were added into the lubricant thin film either with or without deep UV irradiation. A test involved burnishing the magnetic head on the lubricated magnetic hard disk was conducted. The experiment was conducted in a class 100 cleanroom. Contrary to previous studies, the addition of additives into the lubricant film did not lead to a decrease in the amount of wear to the magnetic head. Without deep UV irradiation, the lubricant film combined with additives causes more wear to the magnetic head. The effects of using different percentages of cyclotriphosphazene based additives in perfluoropolyether lubricant were also discussed in this paper. We conclude that deep UV irradiation needed if additives were added when the total lubricant thin film thickness is at 1 nm or below.

     

Theoretical analysis of evaporation and re-adsorption of PFPE molecules on hard disk surface

The collision density of the evaporated lubricant molecules in the air with the hard disk was analyzed based on the kinetic theory of gases. It is found that the collision density is quite high even for the ultralow vapor pressure of the lubricant perfluoropolyether used in hard disks. This suggests that the adsorption of the collided molecules may have significant contribution to the lubricant film replenishment. In addition, the adsorption of the lubricant molecules is also expected to reduce the potential barrier to the surface diffusion, and hence to promote the spreading process of lubricant film on hard disk surface. An adsorbing–spreading mechanism is proposed for the lubricant film replenishment in hard disks.     

Interactions between nano-spacing flying head sliders and ultra-thin liquid lubricant films with non-uniform distribution in hard disk drives

This paper describes the effect of ultra-thin liquid lubricant films on air bearing dynamics and flyability of less than 10 nm spacing flying head sliders in hard disk drives. In particular, the effect of non-uniform lubricant film distributions on head/disk interface dynamics are studied. The disks with lubricant on one half of disk surface thicker than the other half were used in this study. The dynamics of sliders is monitored using acoustic emission (AE) and the interactions between the slider and disk are investigated experimentally. The disks were also examined with a scanning micro-ellipsometer before and after each test. Complicated slider responses were observed and clarified. In addition, it was found that the periodic lubricant film thickness modulations or non-uniformity caused by the slider-disk contact interactions could be observed. It is suggested that this lubricant film thickness non-uniformity will be one of the technical issues in order to achieve ultra-low head/disk contact interface of less than 10 nm.     

Micro-rheometry of pressurized lubricants and micro-nanorheology

In the present study, micro-rheometry of pressurized lubricants employing a diamond-anvil pressure cell and a laser confocal displacement sensor of 0.4 μm resolution was shown. High pressure viscosity was obtained up to 2 GPa at 200°C for traction oils and PFPE oils. The linearity between logarithmic viscosity and pressure is confirmed. Viscosity-pressure coefficient α at room temperature was almost twice larger than that at 100°C. α for hard disk oil, Zdol2000, was 13/GPa at 24°C ~ 5/GPa at 150°C and was similar to that of paraffinic mineral oil. The feature of the obtained high pressure volume was different for each oil up to 6 GPa. Zdol2000 was the most compressible of all the sample lubricants and its high pressure refractive index increased about 10% at 4.8 GPa. Zdol2000 remained transparent up to 4.8 GPa under isothermal loading. Some considerations for lubricant’s micro-nanorheology were also mentioned with high pressure lubricant’s rheology.     

Numerical simulation of molecularly thin lubricant film flow due to the air bearing slider in hard disk drives

In this paper we numerically study the evolution of depletion tracks on molecularly thin lubricant films due to a flying head slider in a hard disk drive. Here the lubricant thickness evolution model is based on continuum thin film lubrication theory with inter-molecular forces. Our numerical simulation involves air bearing pressure, air bearing shear stress, Laplace pressure, the dispersive component of surface free energy and disjoining pressure, a polynomial modeled polar component of surface free energy and disjoining pressure and shear stress caused by the surface free energy gradient. Using these models we perform the lubricant thickness evolution on the disk under a two-rail taper flat slider. The results illustrate the forming process of two depletion tracks of the thin lubricant film on the disk. We also quantify the relative contributions of the various components of the physical models. We find that the polar components of surface free energy and disjoining pressure and the shear stress due to the surface free energy gradient, as well as other physical models, play important rolls in thin lubricant film thickness change.     

Device level studies of adaptive optics sliding components in microprojectors

Integrated microprojectors are being developed to project a large image on any surface chosen by users. For a laser-based microprojector, a piezo-electric based adaptive optics unit is adopted in the green laser architecture. Nanolubrication of adaptive optics sliding components is needed to reduce friction and for stick–slip motion. Previous studies of the role of lubricant film thickness in nanolubrication of sliding components in adaptive optics have been carried out in an academic, coupon level fashion and need to be carried out on a device level in order to characterize the role of lubricant film thickness in an actual working device. In this paper, the effect that operating temperatures have on lubricant film thickness, adhesive force and coefficient of friction of used devices is investigated. The results and associated mechanisms are presented and compared with previous coupon level tests to show that the proposed AFM measurement techniques can be employed in other micro devices in which adhesion, film thickness, and coefficient of friction measurements are of interest.     

Corrected expression of the van der Waals pressure for multilayered system with application to analyses of static characteristics of flying head sliders with an ultrasmall spacing

A corrected and approximate expression of the van der Waals pressure for a multilayered system was developed. The conventional expression cannot be applied to a general multilayered system, while the corrected expression can be applied to multilayered system composed of any materials. Applying the corrected expression to the analyses of the static characteristics of an inclined flying head slider, the dependences of the static load-carrying capacity and the static limit minimum spacing on the PFPE film thickness were clarified.     

Modeling laser induced lubricant depletion in heat-assisted-magnetic recording systems using a multiple-layered disk structure

In this paper, we model the depletion of lubricant from a disk surface subject to heating by a scanning laser in a heat assisted magnetic recording (HAMR) system. A multi-layer disk structure is used consisting of the substrate (either glass or aluminum), the CoFe based soft magnetic under- layer, a Ru based intermediate layer, a CoCrPt based recording layer, the diamond-like-carbon layer, and the lubricant film. The thickness and material properties of the different layers are shown to play an important role in the conduction of heat from the top layer to the bottom layer and, consequently, in the lubricant depletion process due to heating by a scanning laser. The results show that it is critical to include realistic multi-layer disk structures in HAMR lubricant depletion modeling.     

The dynamic behavior of ultrathin lubricant films

Molecular dynamics simulations with the Langevin equation using a coarse-grained, bead-spring model were performed to investigate the dynamic properties of nanoscale lubricant films. The simulated spreading profiles of lubricant nanofilms are in qualitative agreement with previous experimental results. The Einstein’s relationship and Green–Kubo formula provide alternative perspectives for describing the diffusive phenomena of lubricant nanofilms. The effects of molecular weights and their distribution, temperature, and film thickness were examined.     

Deposition of Si-DLC film and its microstructural, tribological and corrosion properties

Silicon-incorporated diamond-like carbon (Si-DLC) films were deposited using a bipolar-type plasma based ion implantation and deposition technique, and the effects of Si-incorporation on the microstructural, tribological, anti-corrosion and lubricant bonding properties of the Si-DLC films were investigated. The analysis of Raman spectroscopy exhibited that the sp³ bonds in the DLC film increase due to Si addition. XPS analysis revealed that a thick oxide layer exists on the Si-DLC film surfaces. These explain the high lubricant bonding properties of the Si-DLC films compared to that of the Si-free DLC films. The silicon oxide layer on the Si-DLC film and the transferred silicon oxide layer on the steel ball prevents from the metal/DLC contact between the Si-DLC film and steel ball when sliding, which results in a low friction. Incorporation of Si in DLC films led to significant improvements in the corrosion resistance due to low internal stress and thick insulating oxide layer.     

Fabrication of layered polydimethylsiloxane/perfluoropolyether microfluidic devices with solvent compatibility and valve functionality

The development of multilayer soft lithography methodology has seen polydimethysiloxane (PDMS) as the preferred material for the fabrication of microfluidic devices. However, the functionality of these PDMS microfluidic chips is often limited by the poor chemical resistance of PDMS to certain solvents. Here, we propose the use of a photocurable perfluoropolyether (PFPE), specifically FOMBLIN^® MD40 PFPE, as a candidate material to provide a solvent-resistant buffer layer to make the device substantially impervious to chemically induced swelling. We first carried out a systematic study of the solvent resistance properties of FOMBLIN^® MD40 PFPE as compared with PDMS. The comparison presented here demonstrates the superiority of FOMBLIN^® MD40 PFPE over PDMS in this regard; moreover, the results permitted to categorize solvents in four different groups depending on their swelling ratio. We then present a step-by-step recipe for a novel fabrication process that uses multilayer lithography to construct a comprehensive solvent-resistant device with fluid and control channels integrated with a valve structure and also permitting easy establishment of outside connections.     

Molecular dynamics simulations of diffusion of submonolayer polar liquid lubricant films on solid surfaces

A fundamental understanding of the diffusion phenomena of submonolayer polar liquid films is important for achieving reliable lubrication between moving mechanical parts separated by a nanometer-sized gap. To acquire this understanding, we conducted molecular dynamics (MD) simulations of diffusion phenomena of submonolayer polar perfluoropolyether (PFPE) Zdol films on solid surfaces. To improve the accuracy of these simulations, we developed an all-atom model that includes hydrogen-bond potential and refined atomic charges for Zdol molecules and tested it through MD simulations of spreading of step-shaped submonolayer PFPE films. Our MD simulations reproduced the experimentally observed effects of polar end groups on the diffusion speed and molecular conformation of Zdol. We then conducted MD simulations of self-diffusion of submonolayer Zdol films; these simulations demonstrated that as the thickness of the submonolayer Zdol films decreases, molecular conformation becomes flatter and the self-diffusion coefficient decreases. These changes in molecular conformation partially explain our experimental finding that the spreading of step-shaped submonolayer polar PFPE films slows down with decreasing initial thickness.